It is known from U.S. Pat. No. 2,700,590 to use tanning agents having a high adstringent effect such as the combination of certain vegetable extracts and a condensation product of a phenol sulpho acid with formaldehyde for tanning the skins of teleosteans, sharks and mammals under special consideration of the easy hydrolysis of the fish collagen. This allows the tanning and thus preservation of skin but the leather becomes very rigid similar to reptile leather but without its pliability and softness. Further the descaling and the removal of the keratin and of other protein components of the skin is not mentioned which constitutes a major problem in the tanning of fish skin.
Further according to U.S. Pat. No. 4,379,708 fish skin tanning is carried out by adjusting the pH of the skin to 1.5 to 3.5 and tanning in two steps by using first a diluted liquor below 35.degree. C. and finally a concentrated liquor at a temperature exceeding 35.degree. C., followed by washing and fat liquoring and adjusting to a pH of at least 6, resulting a leather of a certain softness.
However a good tensile strength and a full feel as well as a pliability of the leather product as well as the necessary strength against the effects of the chemical adstringent during the process are not reached.
In French patent No. 729,942 a tanning method for fish skin is disclosed according to which syntans and subsequently vegetable tanning agents or a mixture of both are used followed by chromium tanning. By this method however, neither the softness nor the pliability of the leather is reached that is held to be desirable.
The process for tanning fish skin is quite different from the process of tanning mammalian skin because of the great difference in the molecular structure of the collagen of fish skin on one hand and of mammalian skin on the other hand. Fish collagen has a linear structure contrary to the branched structure of mammalian collagen. This explains the remarkable difference in the bonds which may be tanned with respect to strength against the temperatures of hydrolysis and the tensile strength.
Further there is an important difference between the temperatures of hydrolysis of the skins. With fish skin this value is about 20.degree. C. lower for almost all fish species than that of mammalian skin. This means that all processes prior to tanning must be carried out at 20.degree. C. below the temperature which is recommended for the pretreatment of mammalian skin.